Mortality immediately after parturition and amongst the newborn is high: thus, about 20% of pigs do not survive the first week of life, and about half of that number die because of gastro-intestinal disorders largely due to infection with pathogenic strains of one or more bacteria. Also gastro-intestinal disorders place a heavy strain on those that survive and contribute to poor growth and to poor weight-gain.
Such resistance as the newborn have to the bacterial infection which they are exposed to at birth or immediately after is acquired from the mothers. This protection (known as passive immune protection, because the anti-bodies conferring protection are received by the young animal, not generated by it) is, in modern stock-raising practice, put to severe test. Thus, with pigs, it is usual to bring sows into a common farrowing house a few days before parturition. Because of the change of environment and the trauma of parturition, natural resistance of the sows (especially those having their first litter) to infection is much diminished. In consequence, the pathogenic strains of bacteria such as E.coli that are already present (but in small population) in the gut multiply. When these pathogens are excreted, the newborn young are exposed to infection by them, even before they take their first suck. Fresh arrivals of sows about to farrow, and the piglets they duly bear, are exposed to increasing possibility of infection from pathogens excreted by the dams and litters already in the farrowing house.